1. Field of the Invention
The present invention relates to a deceleration control device of an internal combustion engine.
2. Description of the Related Art
In Japanese Unexamined Patent Application No. 59-142174, the present applicants proposed an engine in which a throttle valve is arranged in the intake passage of the engine and, in addition, a mechanical supercharger driven by the engine is arranged in the intake passage downstream of the throttle valve. A bypass passage is branched from the intake passage upstream of the throttle valve and is connected to the intake passage downstream of the supercharger. A vacuum operated valve is arranged in the bypass passage and opens when the engine is decelerated.
In such an engine, in which the mechanical supercharger is arranged in the intake passage downstream of the throttle valve, the volume of the intake passage downstream of the throttle valve inevitably increases. As a result, when the decelerating operation of the engine is started, and the engine speed then decreases to an idling speed, the level of vacuum in the intake passage downstream of the throttle valve becomes higher than the level of vacuum produced in the intake passage downstream of the throttle valve when a usual idling operation is carried out. That is, when the throttle valve closes in order to decelerate the engine, since the flow area of the intake passage is restricted by the throttle valve, the level of vacuum produced in the intake passage downstream of the throttle valve rapidly becomes high. At this time, an approximately fixed amount of air is continuously fed via the throttle valve into the intake passage downstream of the throttle valve. When the decelerating operation of the engine is started, and the engine speed then decreases, the amount of air fed into the engine cylinders per unit of time gradually decreases. Contrary to this, at this time, an approximately fixed amount of air is continuously fed via the throttle valve into the intake passage downstream of the throttle valve, as mentioned above. Consequently, as the engine speed decreases, the level of vacuum in the intake passage downstream of the throttle valve becomes gradually low and then is recovered to the level of vacuum in a usual idling operation state. As mentioned above, where the decelerating operation is carried out, the level of vacuum in the intake passage downstream of the throttle valve becomes excessively high immediately after the decelerating operation is started. Then, after a short period, the level of vacuum in the intake passage downstream of the throttle valve is recovered to the level of vacuum in a usual idling operation state.
As mentioned above, when the throttle valve is closed in order to decelerate the engine, an approximately fixed amount of air is fed via the throttle valve into the intake passage downstream of the throttle valve. However, this amount of air is small. Consequently, where the intake passage downstream of the throttle valve has a large volume, even if the engine speed decreases after the decelerating operation is started, the level of vacuum in the intake passage is not easily recovered to the level of vacuum in a usual idling operation state. Consequently, where the intake passage downstream of the throttle valve has a large volume, a long time must pass before the level of vacuum in the intake passage is recovered to the level of vacuum in a usual idling operating state. Therefore, where the decelerating operation is carried out and the engine speed rapidly decreases, the level of vacuum produced in the intake passage when the engine speed decreases to an idling speed becomes higher than the level of vacuum in a usual idling operating state. However, if a higher level of vacuum is produced in the intake passage when the engine speed decreases to an idling speed, the amount of air fed into the engine cylinders decreases. This results in a problem in that the engine will stall.
In order to solve this problem, in the engine proposed by the present applicants, when the decelerating operation is carried out, the vacuum operated valve is caused to open and, thus, the amount of air fed into the intake passage downstream of the throttle valve is caused to increase. As a result, the time period for recovering the level of vacuum in the intake passage is shortened so that the level of vacuum produced in the intake passage when the engine speed decreases to an idling speed becomes approximately equal to the level of vacuum in an ordinary idling operating state. In order to shorten the time period for recovering the level of vacuum as mentioned above, it is necessary to feed a relatively large amount of air into the intake passage from the bypass passage via the vacuum operated valve.
However, when the decelerating operation is carried out, if a larger amount of air is fed into the intake passage from the bypass passage, the amount of fuel fed into the intake passage must be increased accordingly. As a result, a problem occurs in that fuel consumption will increase.